In the operation of industrial fuel burners, particularly burners which are used in operations such as food processing, textile processing and other similar types of processing operations it is desirable that all of the fuel entering the combustion chamber be burned off at the shutdown of the burner sequence to avoid the vaporization of unburnt fuels which might contaminate the product being heated by the burner. Many food processing plants and similar processing installations having for many years used natural gas as a fuel for their burners, and no great problem was involved in the shutdown of the burners as the natural gas left little residual product at the shutdown of the main burner. Since the advent of the fuel shortage problems in the world, many of the industrial process burners have been operated on fuel oil as opposed to natural gas. When these burners are operated with fuel oil, the fuel line between the main fuel valve and the burner contains residual oil that can flow to the burner after the burner has been turned off. This fuel than vaporizes, due to the heat within the combustion chamber, and the vaporized fuel causes two types of problems. The first problem is in contamination of the material being processed by the burner, while the second problem is the carbonization of the fuel at the burner nozzle thereby building up carbon deposits that required excessive maintenance to keep the burners in good operating condition.
The recognition of this problem occurred when the burners were switched from natural gas to fuel oil as a source of fuel. In many processing installations the operation of the burners have been manually operated at shutdown to insure that little or no fuel is injected into the hot combustion chamber and thus can be vaporized to contaminate the processed goods or build up as carbon at the burner nozzle. The manual operation of burners is a very impractical and expensive mode of control. This mode of control also does not take into consideration inadvertent losses of flame and unexpected shutdowns that occur.
In the manual operation of this type of burner, the fuel line to the main burner is normally purged of its remaining fuel by the addition of an air pressure to the fuel line while the pilot burner for the device was kept operating on natural gas. The residual oil is purged to the burner and than is burnt off by the operation of the pilot which would than be manually turned off. It can be seen that inadvertent failures, or the inattention of the operator could cause malfunctions in the shutdown of the burner which could destroy or contaminate batches of processed goods.
In order to overcome this problem, some burner operations have been sequenced by a number of time delay relays which would simulate the manual shutdown function, but these arrangements have not been practical due to the cost, complexity, and inability of the systems to provide for abnormal contingencies in the shutdown sequence of burners of this type.